High-strength steel for pipelines having a low turnover rate and high low-temperature toughness

 

(57) Abstract:

The invention relates to the field of metallurgy, particularly ultrahigh-strength with low yield strength steel for main pipe having improved low temperature on the z toughness and weldability in place and a tensile strength of at least 950 MPa, exceeding the X100 API standard. Steel is a type of low-carbon containing highly Mn-Mo-Nb - traces Ti and selectively containing In, Cu, Cr and V if necessary. She has a microstructure containing martensite/banit and ferritic soft/solid mixed structure, which has a ferritic grain size of 20 to 90%. The said ferrite comprises from 50 to 100% cold-worked (processed) ferrite, and the average grain size of ferrite is not more than 5 m, the formulas determine the measure of hardenability of steel during tempering, which varies from 1.9 to 4.0, depending on the chemical composition of the steel. The invention allows to significantly improve the reliability of pipelines, the effectiveness of their implementation and effectiveness of transport in him natural gas, crude oil, oil, etc. can be dramatically improved. 3 s and 5 C.p. f-crystals, 6 PL.

The invention atorney toughness and weldability, which can be widely applied as a welded steel material for pipelines for transportation of natural gas and crude oils and crude oil, various pressure vessels, industrial mechanical equipment, etc.

The strength of the main pipes for pipelines used to transport over large distances crude oils, crude oil and natural gas in recent years is becoming higher and higher due to 1 improve the efficiency of transportation at the expense of higher pressure and 2 improve the effectiveness of the implementation in place by reducing the outer diameter and weight of main pipes. Main pipe having h according to the standard of the American petroleum Institute (AP1) (yield strength of at least 551 MPa and a tensile strength of at least 620 MPa), to the present time put into use, but the need in the main pipe having a higher strength, is becoming stronger and stronger.

Currently done the research methods of producing ultra-high-strength main pipes based on the known production technologies h of main pipes (for example, NKK Engineering Report, No. 138 (1992), pp. 24-31 and The 7th Of least 760 MPa) in accordance with these technologies is limited.

Known for high strength steel for pipelines having a low turnover rate and high low-temperature toughness, containing carbon, silicon, manganese, phosphorus, sulfur, Nickel, molybdenum, niobium, titanium, aluminum, boron, copper, chromium, vanadium, nitrogen, iron and unavoidable impurities, having a microstructure containing martensite, banit and ferrite (JP. 5195057 A).

When receiving a heavy-duty main pipes it is necessary to strike a balance between strength and low-temperature viscosity, viscosity zone heat welding and weldability in place, the weakening and softening of seams, and so on, and accelerating improvements in heavy-duty main pipes (superior X100).

In addition, the task of the invention is to provide a steel for high-strength pipeline tubes, which is a low carbon steel containing Ni-Nb-Mo-traces of Ti added to the mixture, microstructure which includes a mixed soft/rigid structure of ferrite (having an average grain size of not more than 5 μm and containing a specified number of cold-worked ferrite and martensite, banet.

The solution of these problems is due to the fact that high-strength steel for mA is 0,10

Silicon is Not more than 0.6

Manganese - 1,7 - 2,5

Phosphorus - Not more than 0,015

Sulfur is Not more than 0.003

Nickel - 0,1 - 1,0

Molybdenum - 0,15 - 0,60

Niobium - 0,01 - 0,10

Titanium - 0,005 - 0,030

Aluminum is Not more than 0,06

Bor - Up 0,0020

Copper - 1.2

Chrome - 0.8

Vanadium 0.10

Nitrogen - 0,001 - 0,006

Iron and inevitable impurities Rest.

it is a measure of hardenability during tempering in the range from 1.9 to 4.0, depending on the chemical composition of the steel according to the following formula.

P = 2,7 C + 0,4 Si + Mn + 0,8 Cr + to 0.45(Ni + Cu) + (1+ )Mo + V - 1 + ,

where P is the measure of hardenability during hardening steel

conditional parameter that takes into account the influence of B-boron on hardenability provided that takes the value of ---> 0, when ---> B < 3 ppm, and value ---> 1, when ---> B 3 ppm, and she has a microstructure in which the ferrite fraction is from 20 to 90% of the content in the ferrite from 50 to 100% cold-worked ferrite, and the average grain ferrite is not more than 5 μm.

In addition, high-strength steel contains in weight%:

Bor - 0,0003 - 0,0020

Copper - 0,1 - 1,2

Chrome - 0,1 - 0,8

Vanadium - 0,01 - 0,10

It is expedient if the steel further comprises seamo option of performing a high-strength steel for pipelines, with a low turnover rate and high low-temperature toughness, containing carbon, silicon, manganese, phosphorus, sulfur, Nickel, molybdenum, niobium, titanium, aluminum, boron, nitrogen, iron and unavoidable impurities, having a microstructure containing martensite, banit and ferrite, it differs in that it contains these components when following their ratio in wt.%:

Carbon - 0,05 - 0,10

Silicon is Not more than 0.6

Manganese - 1,7 - 2,2

Phosphorus - Not more than 0,015

Sulfur is Not more than 0.003

Nickel - 0,1 - 1,0

Molybdenum - 0,15 - 0,50

Niobium - 0,01 - 0,10

Titanium - 0,005 - 0,030

Aluminum is Not more than 0,06

Bor - Up 0,0003 - 0,0020

Nitrogen - 0,001 - 0,006

Iron and inevitable impurities Rest.

it is a measure of hardenability during tempering in the range from 2.5 to 4.0, depending on the chemical composition of the steel according to the following formula:

P = 2,7 C + 0,4 Si + Mn + 0,45 Ni + mo,

where P is the measure of hardenability during hardening steel, and it has the microstructure in which the ferrite fraction is from 20 to 90% of the content in the ferrite from 50 to 100% cold-worked ferrite, and an average grain size of ferrite is not more than 5 μm.

This high-strength steel dopolnitelnye another option perform high-strength steel for pipelines, with a low turnover rate and high low-temperature toughness, containing carbon, silicon, manganese, phosphorus, sulfur, Nickel, molybdenum, niobium, titanium, aluminum, copper, chromium, vanadium, nitrogen, iron and unavoidable impurities, having a microstructure containing martensite, banit and ferrite, characterized in that it contains these components when following their ratio in weight. %:

Carbon - 0,05-0,10

Silicon is Not more than 0.6

Manganese - 1,7 - 2,5

Phosphorus - Not more than 0,015

Sulfur is not more than 0.003

Nickel - 0,1 - 1,0

Molybdenum - 0,35 - 0,50

Niobium - 0,01 - 0,10

Titanium - 0,005 - 0,030

Aluminum is Not more than 0,06

Copper - 0,8-1,2

Chrome - 0.6

Vanadium 0.10

Nitrogen - 0,001 - 0,006

Iron and inevitable impurities Rest.

it is a measure of hardenability during tempering in the range from 2.5 to 3.5, depending on the chemical composition of the steel according to the following formula:

P = 2,7 C + 0,4 Si + Mn + 0,8 Cr + to 0.45(Ni + Cu) + Mo + V -1,

where P is the measure of hardenability during hardening steel, and it has the microstructure in which the ferrite fraction is from 20 to 90% of the content in the ferrite from 50 to 100% cold-worked ferrite, and the average grain ferrite is not more than 5 μm.


Calcium - 0,001 - 0,006

REM IS 0.001 - 0.020

Magnesium - 0,001 - 0,006

Below the invention is described in more detail.

First will be considered the microstructure of the steel according to the invention.

While achieving ultra-high strength riziv average of at least 950 MPa, the microstructure of the steel material must contain a specified amount of martensite - bainite, and for this ferrite fraction should be between 20 to 90% (or a fraction of the martensite/bainite should be from 10 to 80%). If the ferrite fraction is more than 90 %, the fraction of martensite/bainite becomes too small, and given the strength may not be achieved. Ferrite fraction also depends on the content of C, if the C content exceeds 0.05%, it is difficult to obtain at least 90 % ferrite fraction.

In the steel according to the present invention, the most desirable ferrite fraction is 30 to 80% from the viewpoint of strength and low-temperature viscosity. However, the first ferrite is soft. Therefore, even when the ferrite fraction is 30 to 90% of specified strength (and, in particular, yield strength and nickle cold-worked ferrite is set to from 50 to 100%. The strain hardening (rolling) ferrite improves the yield stress due to dislocation hardening and subgrain strengthening, and at the same time it is extremely effective to improve the junction temperature of Carpi, as will be shown next.

Even the limitations of the structure, as described above, are not sufficient to achieve excellent low-temperature viscosity. To achieve this goal, you need to use the division through the introduction of cold-worked ferrite average grain size for thin ferrite should not exceed 5 meters Should clarify that in ultrahigh-strength steel to the same separation occurs when testing for the impact strength, etc. by introducing texture of cold-worked ferrite and that the transition temperature fracture decreases dramatically. (Separation or separation is a phenomenon or a phenomenon of the layer exfoliation occurring when tested for impact strength, and, as suggested, it reduces spatial triaxial voltage at the remote end of brittle cracks and improves the characteristics of the phase propagation of brittle cracks).

Also found that when the average size of ferritic grains does not exceed 5 m, martensite/bainite is atory transition and the increase of the yield strength.

As described above, the present invention provides a dramatic improvement in the balance of strength and low-temperature viscosity of the mixed hard/soft structure of ferrite martensite/bainite structure in Nb - Mo steel, low-temperature viscosity which in the past was considered to be bad.

However, even if the microstructure of the steel is strictly controlled, as described above, the steel material having the specified characteristics may not be obtained. To perform this task simultaneously with microstructure is necessary to limit the chemical composition.

The following are the reasons for limiting the chemical composition.

The content of C is in the range of 0.05 to 0.10 wt%. Carbon is an extremely effective element for increasing the strength of steel. In order to obtain the specified strength in the ferrite and martensite/bainite mixed hard/soft structure, the content of C should be at least 0,05 %. This is also the minimum amount to consolidate the effect of dispersion hardening by adding Nb and V, and the effect of grinding of crystal grains, and the strength of the area of the weld. If the C content is too high, then the low-temperature viscosity, and basically is hni limit of the C content is set at 0.10%.

Silicon (Si) is added for deoxidation or restore and improve durability. If its content is too high, the HAZ toughness and weldability in place much worse. Therefore, its upper limit is limited to 0.6%. The deoxidation of steel can be satisfactorily carried out by Ti or Al, and Si is not always necessary to add.

Manganese, in essence, is the element for transformation of the microstructure of the steel of the present invention in ferrite and martensite/bainite mixed hard/soft structure and guarantee an excellent balance between strength and low-temperature toughness, and its lower limit is 1.7%. However, if the Mn content is too high, proclaimest steel increases, and therefore not only worsen HAZ toughness and weldability in place, but also accelerated the Central length of the steel slab obtained by continuous casting and deteriorating the low-temperature toughness of the base metal. Therefore, its upper limit is set equal to 2.5%. The preferred Mn content is from 1.9 to 2.1%.

The task of adding Nickel (Ni) is to improve the strength of low carbon steel of the present invention without hudsen deterioration of reinforced structures in comparison with low-temperature toughness in the laminated structure (in particular, in the Central bundle of the slab), and the addition of traces of Ni determined to be effective to improve and also HAZ viscosity. From the point of view of the HAZ toughness, especially effective amount of the additive Ni is more than 0.3%. However, if the amount of additive is too large, it is not only uneconomical, but also affects the HAZ toughness and weldability in place. Therefore, the upper limit is set equal to 1.0%. The addition of Ni is also effective to prevent Cu cracks during hot rolling and continuous casting. In this case, the Ni must be added in a quantity amounting to at least 1/3 of the Cu content.

Molybdenum (Mo) is added to improve proclaimest steel and provide the specified mixed hard/soft patterns. Present in conjunction with Nb, Mo severely limits the recrystallization of austenite during controlled rolling and improves austenitic structure. To achieve this effect, you must add at least 0.15% of Mo . However, the addition of Mo in an excessive amount affects the HAZ toughness and weldability, and its upper limit is set equal to 0.6%.

In addition, the steel according to the present invention contains from 0.01 to 0.10% of Nb and 0.005 to 0,030% Ti as significant is my rolling and refines the crystal grains. It also contributes significantly to the dispersion hardening and tempering and improves the toughness of the steel. If the added quantity of Nb is too large, it has an adverse effect on HAZ toughness and weldability in place. Therefore, the upper limit of 0.10%.

On the other hand, the addition of titanium (Ti), which forms a thin TIN, limits the coarsening of austenite grains during heating of the slab and welding, improves the microstructure and improves HAZ low-temperature toughness of the base metal and HAZ. When the Al content is low (for example, not more than 0.005%), Ti forms an oxide that functions as the center of the inside-ferritic grain structure formation and improves HAZ structure. To achieve this effect, the additive Ti should be at least 0,005%. However, when the Ti content is too high, there is an integration of Ti and dispersion strengthening due to TiC, and low-temperature toughness is degraded. Therefore, its upper limit is set to 0.03%.

Aluminum (Al) is usually contained in steel as a deoxidizer and has an impact on the improvement of the structure. However, if the Al content exceeds 0.06% of the increase of non-metallic inclusions of the type of aluminum oxide and reduces the purity of the steel. Therefore, the Yes you want to add.

Nitrogen (N) forms a TiN, limits the coarsening of austenite grains during heating of the slab and austenitic grain HAZ, improves the low-temperature viscosity as the base metal and HAZ. The minimum required amount in this case is 0.001%. However, when the N content is too large, it will lead to defects on the slab surface, and deterioration of the HAZ toughness due to the formation of solid solution N. Therefore, its upper limit should be limited 0,006%.

In addition, the present invention limits the content of P and S as the impurity element to not more than 0.015 percent and not more than 0,003% , respectively. The main task of these elements is further improved low-temperature toughness of the base metal and HAZ. The reduction in the content of P reduces the Central bundle is continuously featured slab, prevents the destruction of grain boundaries and improves the low-temperature viscosity. The reduction in the content of S is necessary in order to reduce the MnS, which is stretched and lengthened under controlled rolling and improves the ductility and toughness.

Moreover, if necessary, selectively added to at least one of the following items:

B : from 0.0003 to 0.00 CLASS="ptx2">

Boron (B) limits the formation of large ferrite at the grain boundaries during the rolling process and contributes to the formation of thin small ferrite inside the grains. In addition B limits the formation of intergranular ferrite in the HAZ and improve HAZ toughness in welding with a large heat input such as SAW (arc welding in a protective atmosphere), used roller welding steel pipes. If the number of added B is not more than 0,0003%, the effect cannot be achieved, and if it exceeds 0,0020 %, will be allocated to the boron compounds that will lead to lower low-temperature viscosity. Therefore, the amount of additive of boron is in the range from 0.0003 to 0,0020%.

Copper (Cu) dramatically improves the strength of ferrite and martensite/bainite two-phase mixed structure due to hardening and dispersion hardening martensite/bainite phase. It is also effective to improve corrosion resistance and retinologist from cracking caused by hydrogen (resistance to hydrogen embrittlement). If the copper content is less than 0.1%, these effects are not achieved. Therefore, the lower limit is 0.1%. Adding excessive amounts of copper leads to the decrease of the viscosity and of the base metal and the HAZ due to the dispersed SS="ptx2">

Chromium (Cr) increases the strength of the area of the weld. If the number of added chromium is too great, the HAZ toughness and weldability in place noticeably deteriorates. Therefore, the upper limit of the chromium content is 0.8%. If the number of added chromium is less than 0.1%, these effects cannot be achieved. Therefore, the lower limit is 0.1%.

Vanadium (V) has essentially the same effect as Nb, but its effect is weaker than that of Nb. However, the effect of the additive V in ultra-high-strength steel is fairly large and complex additive Nb and V makes excellent characteristics of the present invention is even more noticeable. V susceptible to deformation-induced selection is in process (hot rolling) ferrite and significantly strengthens the ferrite. If the amount of additive of vanadium is less than 0.01%, this effect cannot be achieved. Therefore, the lower limit is 0.01%. The upper limit can reach up to 0.10% from the viewpoint of the HAZ toughness and weldability in place, with the most preferred range is from 0.03 to 0.08%.

In addition, if necessary, can be added to the following components:

Ca: 0.001 to 0.006%

REM: 0.001 to 0.02%,

or at least one of Ca, Rare-earth metals, Mg and y

Ca and REM regulate the formation of sulfide (MnS) and improve the low-temperature viscosity (increase the energy absorption tests for impact strength, etc. But cannot be achieved any practical result, if the content of Ca or REM is not more than 0,001 %, and if the Ca content exceeds 0,006 % or the content of REM exceed 0.02 %, produces large quantities of CaO-CaS, or REM-CaS that has resulted in large clusters and large inclusions. They not only degrade the cleanliness of the steel, but is also harmful influence on welding on site. Therefore, the upper limit of the added amount of Ca or REM is set to 0.006% or 0.02%, respectively. In addition, ultra-high-strength main pipes is especially effective in reducing the content of S and About 0.001% to 0.002%, respectively, and the regulation values ESSP from ESSP = (Ca) [1 - 124(0)]/1,25 S to 0.5 ESSP and 10.0. The term ESSP is an acronym for "Effective Sulfide State control Parameter (parameter regulation effective sulfide).

Each of the two elements, i.e., magnesium (Mg) and yttrium (Y) form a minor oxides, limit the growth of the grains, when the steel is rolled or heat, and improve the structure after hot rolling. In addition, they inhibit the growth zone and the effects can not be achieved, and if the number of the additive is too large, then they become major oxides and deteriorate the low-temperature viscosity. Therefore, the amount of additive set for Mg from 0.001 to 0.006%, and for Y from 0,0012 to 0.10%. When you add Mg and Y, the Al content is preferably set to not more than 0.005% from the point of view of the fine dispersion and fluidity.

In addition to the limitations of the additive of the individual elements described above, the present invention preferably limits the value of

P = 2,7 C + 0,4 Si + Mn + 0,8 Cr + to 0.45(Ni + Cu) + (1 +)Mo + V -1

to 1.9 P 4,0, when the steel contains Mo basis, up to 2.5 P 4,0, when additionally add B, and up to 2.5 P 3,5, when the steel is further added Cu. This is done to obtain a given balance between strength and low-temperature toughness without compromising the HAZ toughness and weldability in place. The lower limit value P is set equal to 1.9 in order to obtain a strength of at least 950 MPa and excellent low-temperature toughness. The upper limit value P is set equal to 4.0 in order to maintain excellent HAZ toughness and weldability in place.

In the present invention steel type low C - high Mn - Nb - V - Mo - Ti, and type Ni - Mo - Nb - traces of Ti - traces B and type Ni - Cu - Mo - Nb - traces T is aznoe austenite/ferrite zone and cooled air or cooled rapidly by air to obtain a thin cold-worked ferrite plus martensite/bainite in the mixed structure, consequently simultaneously achieves ultra-high strength and excellent low-temperature toughness and weldability and the softening of the area of the weld due to the mixed structure of cold-worked ferrite plus martensite/beinit. I will then explain the reasons for the restrictions of the conditions of production.

In the present invention, the first slab is heated to a temperature in the range from 950 to 1300oC and then rolled in a hot condition to the cumulative degree of compression during rolling was at least 50% at a temperature of not higher than 950oC coefficient or the degree of cumulative compression ratio ranged from 10 to 70%, preferably from 15 to 50% in the ferrite-austenite two-phase zone from point Ar3to the point Ar1and the final temperature rolling in a hot condition ranged from 650 to 800oC. then laminated in a hot state plates cooled air or cooled with a cooling rate of at least 10oC/s to an arbitrary temperature not higher than 500oC.

This process aims to preserve a small initial austenite grains during heating of the slab and the improvement of the laminated structure. Than m the Rita-martensite. Temperature 1300oC is the upper limit of the temperature at which austenite grains during heating does not become large. On the other hand, if the heating temperature is too low, the alloying elements do not dissolve sufficiently, and a given material cannot be obtained. Because you need the heat for an extended period of time in order to evenly heat the slab, and the deformation resistance during rolling in a hot state becomes large, undesirable increases energy consumption. Therefore, the lower limit of the heating temperature set temperature of 950oC.

Heated slab should be laminated so that the amount of cumulative compression during rolling at a temperature not exceeding 990oC was at least 50%, cumulative narrowing of the two-phase ferrite-austenitic zone from the point Ar3to the point Ar1ranged from 10 to 70%, preferably from 15 to 50%, and the final temperature of the rolling ranged from 650 to 800oC. the Reason why the cumulative compression below 950oC limited value constituting at least 50%, increase prokachivanija in the austenitic nareki the Les transformations in mixed ferrite - martensite/bainite structure. Ultrahigh-strength line pipe having a tensile strength of at least 950 MPa, requires a higher viscosity than even from the point of view of security. Therefore, it is the cumulative compression should be at least equal to 50%. (The value of cumulative compression during rolling, it is preferable, is as high as possible, and no upper limit).

In addition, in the present invention, the amount of cumulative compression or narrowing of the ferrite-austenite two-phase zone should be equal to from 10 to 70%, and the final temperature of the rolling should be between 650 to 800oC. This is done in order to further grind the austenite in precrystallization area to negativity and to strengthen the ferrite and provide easier separation during tests on the impact strength.

When the amount of cumulative narrowing when rolling two-phase zone is below 50%, is the lack of separation, and cannot be achieved by improving the characteristics of the stop the propagation of brittle cracks.

Even when the amount of cumulative narrowing during rolling is acceptable, excellent is or unacceptable. If the final temperature rolling in a hot condition below 650oC becomes significant embrittlement of the ferrite due to mechanical processing. Therefore, the lower limit of the end of rolling temperature in hot condition is set to 650oC. If the final temperature rolling in a hot condition exceeds 800oC, grinding austenitic structure and providing the separation are insufficient. Therefore, the upper limit end of rolling temperature in hot condition is limited to 800oC.

After rolling hot steel plates or cooled air or cooled to an arbitrarily selected temperature below 500oWith speeds of cooling component at least 10oC/min In the steel of the present invention mixed ferrite and martensite/bainite structure can be obtained, even when the cooling is carried out after rolling, but in order to further increase the strength, the steel plates can be cooled to an arbitrarily selected temperature below 500oC with a cooling rate of at least 10oC/s Cooling at a rate equal to at least 10oC/s accelerates turned the temperature and stop the cooling water above 500oC, improve balance, strength and low-temperature viscosity due to hardened sufficiently not to be expected.

One of the distinctive characteristics of the steel of the present invention is that there is no need to leave steel, although the leave may be carried out in order to perform the cooling.

Option:

The following describes examples of the present invention.

Example 1. Slabs, having different chemical composition was obtained by melting at laboratory scale (50 kg ingot with a thickness of 120 mm) or by way Converter-continuous casting (thickness 240 mm). These slabs were rolled in a hot condition in a thick steel sheet with thickness from 15 to 32 mm under various conditions, and then explored various mechanical properties and microstructure. (Some thick steel sheets were subjected to vacation).

Mechanical properties of thick plates (yield strength : YS, tensile strength TS, the energy absorption at -40oC in the testing of Charpy on impact strength: vE-40the transition temperature of 50% destruction: vTrs) were investigated in a direction at right angles to the direction of rolling.

HAZ toughness (energy absorption at -20ooC, the cooling time from 800 to 500oC [ t800-500]: 25 seconds).

The weldability was evaluated by the lower preheating temperature required to prevent low-temperature cracking of HAZ in the tests on the V - slit weld crack (JIS G 3158 ) (method of welding : gas metal arc welding electrode, electrode (welding rod): compressive strength of 100 MPa, heat input: 0,5 kJ/mm, the amount of hydrogen in the weld metal: 3 CC/100 g metal).

The test results of examples are shown in tables 1 and 2.

Sheet steel, obtained in accordance with the method of the present invention, had excellent balance between strength and low-temperature toughness, HAZ, toughness and weldability in place. In contrast, steel for comparison had a significantly worse performance of any of their properties due to the mismatch of their chemical compositions and microstructures.

Because steel N 9 had excessive levels of C, the energy absorption of Care and in the base metal and HAZ was low, and the temperature of the heated during welding was high. Because steel N 13 did not add Mb, strength was unsatisfactory, the grain size of the ferrite was ballyeamon viscosity and base metal, and HAZ was low. Because steel N 18 the grain size of the ferrite was too large, low-temperature toughness was significantly lower. Since the ferrite fraction and the fraction of cold-worked ferrite in steel No. 19 were too small, the yield was low and the transition temperature of Capri was low.

Example 2. Slabs having various chemical compositions were obtained by melting in a laboratory scale bar 100 kg 150 mm thick) or by way Converter-continuous casting (thickness 240 mm). These slabs were laminated in a hot condition to a thick steel plates with thickness from 16 to 24 mm under various conditions, and then researched various mechanical properties and microstructure (yield strength : YS, tensile strength TS, the energy absorption at -40oC in the testing of Charpy: vE-40the transition temperature of 50% destruction : vTrs in the direction at right angles to the direction of rolling. Index separation at the turn of Carpi if -100oC (the value obtained by dividing the total length of the separation fracture in the area of 8x10 (mm2) break; the higher the value, the more beautiful features of halting the spread of cracks ) was measured as a characteristic stop by radiostream simulate the heating temperature HAZ (maximum temperature of 1400oC, the cooling time from 800 to 500oC [ t800-500]: 25). The weldability was evaluated by the lower preheating temperature required to prevent low-temperature cracking of HAZ in the testing of the Y-slit weld crack (JIS G3158) (method of welding : gas arc welding with a metal electrode, the electrode (welding rod): the yield strength of 100 MPa, heat input: 0.3 kJ/mm, the amount of hydrogen in the weld metal: 3 CC /100 g metal).

In table. 3 and 4 show samples and measurements of their characteristics.

Steel plate obtained in accordance with the method of the present invention, showed excellent balance of strength and low-temperature viscosity and excellent HAZ toughness and weldability in place. In contrast, in the steels for comparison, because of the chemical composition and microstructure were unacceptable, all their characteristics have been much worse.

Example 3. Slabs having various chemical compositions were obtained by laboratory melting (50 kg ingot with a thickness of 100 mm) or by way Converter-continuous casting (with a thickness of 240 mm). These slabs are rolled in a hot condition to a thick steel plates with a thickness of 15 to 25 mm prucher.

Various mechanical properties of thick sheet steel (yield strength: YS, tensile strength TS, the energy absorption at -40oC in the testing of Charpy: vE-40the transition temperature of 50% destruction : vTrs) were investigated in a direction at right angles to the direction of rolling.

HAZ toughness (energy absorption at -40oC in the testing of Charpy: vE-40) was estimated by simulating samples (maximum temperature of 1400oC, the cooling time from 800 to 500oC [ t800-500]: 25 seconds).

The weldability was evaluated by the lower preheating temperature required to prevent low-temperature cracking of HAZ in the testing of the Y - slit weld crack (JIS G3158) (method of welding : gas arc welding with a metal electrode, the electrode (welding rod) : compressive strength of 100 MPa, heat input : 0.3 kJ/mm, the amount of hydrogen in the weld metal: 3 cu cm/100 g of metal).

These examples are given in table. 5 and 6.

Steel plate obtained according to the method of the present invention, showed excellent balance of strength and low-temperature viscosity and excellent HAZ toughness and weldability in place, unlike it was obvious the systematic composition and microstructure were not relevant.

The effect of the invention.

The present invention can provide stable mass production of ultrahigh-strength steel for main pipe (having a tensile strength of at least 950 MPa and greater than the X100 on Estandarte), which has excellent low-temperature toughness and weldability in place. As a result, the reliability of the pipeline can be significantly increased and the transfer efficiency and the effectiveness of the execution pipeline can be dramatically increased.

1. High-strength steel for pipelines having a low turnover rate and high low-temperature toughness, containing carbon, silicon, manganese, phosphorus, sulfur, Nickel, molybdenum, niobium, titanium, aluminum, boron, copper, chromium, vanadium, nitrogen, iron and unavoidable impurities, having a microstructure containing martensite, banit and ferrite, characterized in that it contains these components in following them, the weight. %:

Carbon - 0,05 - 0,10

Silicon is Not more than 0.6

Manganese - 1,7 - 2,5

Phosphorus - Not more than 0,015

Sulfur is Not more than 0.003

Nickel - 0,1 - 1,0

Molybdenum - 0,15 - 0,60

Niobium - 0,01 - 0,10

Titanium - 0,005 - 0,030

BR>
Iron and inevitable impurities - Rest

it is a measure of hardenability during tempering in the range from 1.9 to 4.0, depending on the chemical composition of the steel according to the following formula:

P = 2,7 C + 0,4 Si + Mn + 0,8 Cr + to 0.45(Ni + Cu) + (1 + ) Mo + V - 1 + ,

where P is the measure of hardenability during hardening steel;

- conditional parameter that takes into account the influence of B - boron on hardenability provided that takes the value --> 0, when --> B < 3 ppm, and the value --> 1, when --> B 3 ppm, and she has a microstructure in which the ferrite fraction is from 20 to 90% of the content in the ferrite from 50 to 100% cold-worked ferrite, and the average size of the ferrite is not more than 5 m

2. High-strength steel under item 1, characterized in that it contains, in weight. %:

Bor - 0,0003 - 0,0020

Copper - 0,1 - 1,2

Chrome - 0,1 - 0,8

Vanadium - 0,01 - 0,10

3. High-strength steel according to any one of paragraphs.1 and 2, characterized in that it further comprises the following components, wt.%:

Calcium - 0,001 - 0,006

REM IS 0.001 - 0.020

Magnesium - 0,001 - 0,006

4. High-strength steel for pipelines having a low turnover rate and high low-temperature toughness, containing carbon, silicon is the real microstructure, containing martensite, banit and ferrite, characterized in that it includes the components the next time their attitude, wt.%:

Carbon - 0,05 - 0,10

Silicon is Not more than 0.6

Manganese - 1,7 - 2,2

Phosphorus - Not more than 0,015

Sulfur is Not more than 0.003

Nickel - 0,1 - 1,0

Molybdenum - 0,15 - 0,50

Niobium - 0,01 - 0,10

Titanium - 0,005 - 0,030

Aluminum is Not more than 0,06

Bor - 0,0003 - 0,0020

Nitrogen - 0,001 - 0,006

Iron and inevitable impurities - Rest

it is a measure of hardenability during tempering in the range from 2.5 to 4.0, depending on the chemical composition of the steel according to the following formula:

P = 2,7 C + 0,4 Si + Mn + 0,45 N + 2Mo,

where P is the measure of hardenability during hardening steel, and it has the microstructure in which the ferrite fraction is from 20 to 90%, with the content in the ferrite from 50 to 100% cold-worked ferrite and an average grain size of ferrite is not more than 5 m

5. High-strength steel under item 4, characterized in that it further comprises the following components, wt.%:

Vanadium - 0,01 - 0,10

Chrome - 0,1 - 0,6

Copper - 0,1 - 1,0

6. High-strength steel for pipelines having a low turnover rate and increased low-temperature Vya is m, vanadium, nitrogen, iron and unavoidable impurities, having a microstructure containing martensite, banit and ferrite, characterized in that it includes the components the next time their attitude, wt.%:

Carbon - 0,05 - 0,10

Silicon is Not more than 0.6

Manganese - 1,7 - 2,5

Phosphorus - Not more than 0,015

Sulfur is Not more than 0.003

Nickel - 0,1 - 1,0

Molybdenum - 0,35 - 0,50

Niobium - 0,01 - 0,10

Titanium - 0,005 - 0,030

Aluminum is Not more than 0,06

Copper - 0,8 - 1,2

Chrome - 0.6

Vanadium 0.10

Nitrogen - 0,001 - 0,006

Iron and inevitable impurities - Rest

it is a measure of hardenability during tempering in the range from 2.5 to 3.5, depending on the chemical composition of the steel according to the following formula:

P = 2,7 C + 0,4 Si + Mn + 0,8 Cr + to 0.45(Ni + Cu) + Mo + V - 1,

where P is the measure of hardenability during hardening steel, and it has the microstructure in which the ferrite fraction is from 20 to 90% of the content in the ferrite from 50 to 100% cold-worked ferrite and an average grain size of ferrite is not more than 5 m

7. High-strength steel under item 6, characterized in that it contains, in weight. %:

Chrome - 0,1 - 0,6

Vanadium - 0,01 - 0,10

8. High-strength steel according to any one of paragraphs.4 to 7, characterized in that it d ,006

Priority points:

06.02.95 on PP.1 - 3;

31.07.95 on PP.4, 5, 8;

03.02.95 on PP.6 and 7.

 

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